This invention relates to semiconductor electronic devices, and, more particularly, to high electron mobility transistors.
Increasing the performance (i.e., smaller propogation delays, larger current, larger band widths, etc.) of semiconductor devices such as silicon field effect transistors (FETs) can be approached in at least two ways: (1) scaling down the size of the devices and (2) increasing the carrier channel mobility. With regard to the second approach, substituting gallium arsenide for silicon may increase mobility at room temperature by a factor of about six; but, more importantly, using gallium arsenide/doped aluminum gallium arsenide heterojunction structures can yield a further gain in mobility, especially at liquid nitrogen temperatures. See, generally, R. Eden, Comparison of GaAs device approaches for ultrahigh-speed VLSI, 70 Proceedings of the IEEE 5 (1982). Such heterojunction structures consist of an n-type doped layer of AlGaAs joining an undoped GaAs layer; the wider band gap of the AlGaAs leads to the donated electrons transfering into the conduction bands of the GaAs, which are at lower energies, and leaving behind the donor ions in the AlGaAs. This separation of the donated electrons from the donor ions significantly reduces impurity scattering and enhances mobility, particularly at lower temperatures.
Heterojunctions generally and AlGaAs/GaAs in particular can be grown by liquid-phase epitaxy (LPE), molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD). MBE and MOCVD are preferably to LPE and can grow alternating epitaxial layers of AlGaAs and GaAs with layer thicknesses below 50A for MOCVD and below 10A for MBE. However, successful heterojunction devices are invariably fabricated with the doped AlGaAs being epitaxially grown on the undoped GaAs, and this is called the "normal" structure. Conversely, the "inverted" structure is with the undoped GaAs epitaxially grown on the doped AlGaAs. The drawbacks of the normal structure include the need to make a rectifying gate contact (Schottky barrier) on the rapidly oxidizing AlGaAs, and having the AlGaAs on top in these devices makes them more susceptible to degradation. For these reasons, the inverted structure is preferable; also the inverted structure provides better electron confinement. Nevertheless, the potential advantages of the inverted structure have been offset by difficulties in obtaining the mobility enhancement that has been obtained in the normal structure. See, R. Thorne et al, Performance of Inverted Structure Modulation Doped Schottky Barrier Field Effect Transistors, 21 Jap. J. App. Phy. L223 (1982). Indeed, the poor performance of inverted structure devices shows up as a low transconductance and a failure to pinch off.
One of the limitations of the normal structure device (typically called a "high electron mobility transistor" or HEMT) is the fairly low current capability compared to a standard GaAs FET. The obvious solution of growing multiple alternating layers of AlGaAs and GaAs to form a sort of multi channel HEMT would appear to overcome this current limitation problem; however, as just discussed, the inverted structures alternating between the normal structures would lead to poor performance for such a multi channel HEMT. Consequently, it is a problem in the prior art to fabricate a normal structure HEMT with large current capability.